The present invention relates to electrical measuring systems. The invention is particularly useful for making certain biological measurements useful in the non-invasive examination of biological tissue, and is therefore described below with respect to such applications.
One such application described below, for which the invention is eminently suitable, is the impedance measurement of sections of biological tissue, e.g., to provide an indication of the condition of such tissue. A second application, also described below, is in monitoring the muscle-tension of a subject, e.g., in measuring the jaw bite of the subject.
The impedance measurement of sections of biological tissue, by providing an indication of the condition of such tissue, allows the non-invasive monitoring of changes associated with such tissue, particularly in the subdermal concentration of fluids. For example, a wound, burn or skin graft tends to exhibit an impedance which is inversely proportional to the accumulation of biological fluids, and therefore the monitoring of the impedance of the biological tissue, particularly the dermis or deep-tissue impedance, provides a means, which is not disturbing, nor intrusive nor cumbersome to the subject, for monitoring the progress and state of the wound, burn or skin graft.
One simple system for measuring the electrical impedance of biological tissue includes a source of constant-amplitude current, a pair of electrodes applied across the portion of the tissue under examination, and means for measuring the voltage drop across the pair of electrodes, thereby providing a measurement of the impedance in the respective portion of the biological tissue under examination. However, such a "two-electrode" system cannot distinguish between the dermis or deep-tissue impedance, which is of the primary interest, from either the contact impedance or the epidermis impedance, and therefore such "two-electrode" systems are not very sensitive. Three-electrode systems and four-electrode systems have been devised to increase the sensitivity of the system in the above respects, but these too are still not as sensitive as would be required for effectively monitoring the condition of the tissue by observing its change in impedance.
For example, a four-electrode system includes a first pair, constituting excitational electrodes, adapted to be applied in spaced relationship to each other to the tissue under examination and to be connected to the source of constant-amplitude current; and a second pair, constituting sensing electrodes, adapted to be applied to the subject under examination in alignment with, and in the space between, the excitation electrodes and in spaced relationship to each other. Since a constant current is applied to the tissue under examination, the voltage measured by the sensing electrodes provides a measurement of the impedance between them, and thereby, of the impedance in the tissue under examination. Thus, the measurement produced by the four-electrode system (i.e., the voltage drop between the sensing electrodes) does not include either the contact-impedance or the epidermis impedance measurements. However, while such a four-electrode system provides an output closer to the actual dermis or deep-tissue impedance than the mentioned two-electrode system (or three-electrode system), it too is not exact, as will be explained more fully below.
One object of the present invention is to provide an electrical measuring system which, when used for measuring the impedance of biological tissue, provides an even more accurate measurement, than the four-electrode system, of the impedance of primary interest, namely the dermis or deep-tissue impedance.
Certain features of the electrical measuring system of the present invention may advantageously be used in making and evaluating other electrical measurements, such as in monitoring the muscle-tension of the subject.
Accordingly, another object of the present invention is to provide an electrical measuring system which may be used in measuring not only electrical impedance but also muscle-tension, temperature, or other conditions, in a manner so as to more readily facilitate, to the observer, an evaluation of the significance of such measurements.